In known heating and/or air conditioning installations for motor vehicles, in order to enable an occupant of a vehicle to set the atmospheric parameters required in the cabin, the control system of the installation has at least two, and more usually three, manually controlled members which are typically in the form of rotary knobs. Because of the number of these control members, the control panels are difficult to read, that is to say it is difficult to absorb the current state of the system at a glance, and difficult to see at a glance what adjustments to the control members need to be made if a change is to be made in the atmospheric conditions. In general, this makes the conventional controls of vehicle heating, ventilating and air conditioning systems somewhat inconvenient to use. In addition, the controls occupy quite a large amount of space, and this gives rise to problems in some vehicles, especially small vehicles intended for urban use.